This invention relates to metallic protective coatings for uranium. It is particularly concerned with bronze coatings on uranium for protection of the uranium against corrosion and for use under other metal coatings.
Metallic uranium is highly reactive with oxidizing agents and its use in the presence of air or other oxidizing media requires its protection by some less reactive coating. The metallic uranium has a tendency to alloy with some coating metals, especially at ele-vated temperatures, and this tendency may lead to diffusion of uranium through the coating metal with consequent reduction in resistance or the coating to oxidizing agents.
In the accompanying drawings, FIGS. 1-3 are photomicro-graphs of cross-sectional views of uranium articles coated by the process of this invention.
The present invention has for its object the provision of coatings which are not only resistant to oxidizing agents and other corrosive media but are also resistant to the diffusion of uranium and consequently retain their protective value over long periods and under widely varying conditions of exposure.
In accordance with the present invention metallic uranium is provided with a protective coating of bronze. The coating preferably is applied by dipping the metallic article into molten bath of the coating metal.
The proportions of copper and tin in the coating may be varied over a wide range. In general it is desirable that the coating contain about 20-75% copper and about 80-25% tin by weight. Other metals may be present in minor proportions. Coating baths having the composition of speculum metal (67% copper, 33% tin) have been employed satisfactorily to provide continuous coatings of this metal. Bronze baths containing copper and tin corresponding to the peri-tectic mixture (47%, copper 53% tin) also have been employed and these baths have been found to be especially advantageous in the application of undercoatings for the application of certain other coating metals by the hot dip process. In the early application of bronze coatings to uranium, it was found that the addition of about 1% of aluminum or xc2xd% to 5% of nickel was advantageous for improving the continuity of the resulting coatings. Later it was determined that, by employing uranium the surface of which has been properly prepared, no additions to the bronze baths are necessary to produce continuous, adherent coatings. A satisfactory preparatory treatment comprises immersing the uranium in 50% to 70% nitric acid at a temperature between 50xc2x0 and 60xc2x0 C. for between four and six minutes, rinsing the metal in clean, warm water, and drying immediately before dipping in the molten bronze bath.
The molten bronze bath may be emp1oyed with a dry surface but is preferably protected by an alkali-metal chloride flux, as described and claimed in U.S. patent applicatiorn Ser. No. 583,176 filed Mar. 16, 1945 by Lowell D. Eubank.
The optimum temperature for bronze-dipping the metallic uranium depends upon the composition of the bronze being applied. Temperature from 700xc2x0 to 850xc2x0 C. have been employed with com-position varying from the peritectic mixture to speculum metal.
The bronze coatings may serve as temporary or permanent protective coatings in the application of coating metals such as zinc, tin, terne, aluminum-silicon alloys, and aluminum. Tin does not coat the speculum metal coatings readily and con-sequently when this metal is employed it is desirable to employ a bronze having a composition nearer that of the peritectic mixture or to follow the dip in speculum metal with a dip in a bronze bath containing a greater tin concentration before applying the pure tin coating. Two-dip coatings of copper-tin peritectic over speculum metal present a more workable surface than the speculum metal alone. Coatings prepared from bronze baths containing 57% copper and 43% tin by weight exhibit properties similar to the coatings applied by dipping first in speculum metal and then in the peritectic mixtures.
The bronze coatings of the invention have been found to be especially suitable as temporary protective undercoatings for the application of aluminum-silicon casting alloy coatings or brazings to uranium. Aluminum-silicon casting alloys are aluminum alloys of silicon in which the aluminum predominates. The principal alloys of this type are those containing 5% to 20% by weight of silicon and the remainder essentially aluminum. The ternary alloys of aluminum, silicon, and sodium with about 10-15% silicon and about 0.1% sodium are commonly preferred. All of these alloys tend to form with metallic uranium a layer of a brittle compound of aluminum, uranium, and silicon at the interface between the metal and the aluminum-silicon coating. This compound layer frequently exhibits cracks which increase in size with the thickness of the compound layer. A compound layer of substantial thickness is undesirable not only because of its characteristic brittleness but because the presence of cracks and fissures substantially impairs the protective value of the coating. By the application of a bronze undercoating prior to dipping metallic uranium in aluminum-silicon alloy, the formation of the brittle compound layer may be inhibited and its thickness held to a value such that the objectionable characteristics do not attain significant proportions.
Bronze undercoats beneath aluminum-silicon alloy coatings have a highly important value in substantially preventing undercutting by acidic water containing oxidizing agents and chloride ions. Thus an aluminum-silicon alloy coating may be completely penetrated and yet continue to afford adequate protection for the underlying metal. This property is characteristic even though the final product contains a copper film only 0.01 to 0.03 mil thick and considerably denuded of tin, beneath the aluminum-silicon alloy.
Aluminum-silicon alloys do not wet bronze coatings on uranium readily and dipping times as long as 30 seconds may be necessary to secure continuous coatings by this method. Such long coating periods may lead to washing off of the bronze and formation of substantial areas of thick, brittle compound layer. By subjecting the bronze-coated metal to a preliminary dip in molten tin, the time required for coating with molten aluminum-silicon alloys may be reduced to a fraction of that required for applying the aluminum-silicon coatings directly to the bronze-coated metal. Thus instead of a 30-second dipping period only a 1- to 5-second dip is necessary.
In certain applications the presence of tin in the outer aluminum-silicon coatings decreases the corrosion resistance of the coatings. For such applications it is desirable to remove excess tin by wiping or centrifuging the tin-coated article immediately after the tin dip so as to remove all excess molten metal, and to renew the aluminum-silicon bath whenever its tin content attains an undesirably high value. Centrifuging may be effected in a basket type or lathe type centrifuge supporting the article either concentrically or eccentrically and using speeds corresponding to force from 100 to 10,000 or more times the force of gravity.
Application of the tin coating at a high temperature, for example a temperature of at least 600xc2x0 C. and preferably 630-640xc2x0 C. also is instrumental in providing an exceedingly thin tin coating. This also is necessary for maintaining the uranium article in Beta phase in those cases in which reversion to the Alpha phase would be objectionable. Uranium passes from Alpha to Beta phase when heated above about 650xc2x0 C. but re-version to Alpha phase during normal coating periods may be inhibited by maintaining temperatures above about 600xc2x0 C.
By applying a bronze coating from a bath containing about 45-50% copper and about 50-55% tin at a temperature of about 730xc2x0 C., a tin coating at about 600xc2x0 C. with subsequent centrifuging in a 12-inch basket at about 640 rpm maximum speed for 5 seconds, and then an aluminum-silicon coating of about 88 parts aluminun and 12 parts silicon at 640xc2x0 C. for 2 seconds, coatings are obtained which are free from objec-tionable compound layers and which contain copper and tin in amounts corresponding to bronze 0.03 to 0.1 mil thick between the uranium metal and the aluminum-silicon coatings. Longer dips in the aluminum-silicon alloy produce thinner bronze layers of reduced tin content and the preferred procedure in-volving a dipping period of about 6 seconds in aluminum-silicon provides a final product with a cuprous film only 0.01 to 0.03 mil thick of very low tin content.
The tin bath and the aluminum-silicon bath may be employed without top fluxes. A top flux on the aluminum-silicon bath has the disadvantage that, upon cooling, the flux crystals may mark the coating, and when an aluminum sheath is applied over the aluminum-silicon, some of the flux may be entrapped between the sheath and the base metal and thus prevent a continuous bond between the two. However by using a dividing partition at the surface of the alloy, it is possible to employ a flux on part of the aluminum-silicon in such a manner that the article dipped therein can be removed through a flux-free surface, thereby avoiding any adhesion of flux to the metal article. This arrangement can be used in conjunction with the flux on the bronze bath in such a manner as to eliminate an inter-mediate tin dip. Thus by providing a flux bridge (essentially a flux-filled channel) from the bronze bath to the aluminum-silicon bath so that the metal article may be maintained under flux throughout its passage from the bronze bath to the aluminum-silicon bath and can then be withdrawn at a flux-free surface of the aluminum-silicon bath, satisfactory coatings can be obtained. This method permits relatively rapid wetting of bronze coatings by the aluminum-silicon bath but wetting is not as rapid in this case as when an intermediate tin dip is employed.
In employing the bronze, tin, aluminum-silicon sequence, some uranium is dissolved in the bronze bath, some bronze is dissolved in the tin bath, and some tin is dissolved in the alumium-silicon bath. An alkali-metal chloride flux on the surface of the bronze bath has been found to maintain the uranium content of this metal bath at value below 0.07% by weight. For the removal of uranium from bronze baths which are free of flux a small quantity of aluminum may be added. This forms an alloy with the uranium which can he skimmed from the bath. The introduction of copper into the tin bath by solution of bronze from the coated metal can be offset by intermittently adding a small proportion of aluminum at 600-650xc2x0 C., and after thoroughly mixing the added metal with the bath, cooling to 300-350xc2x0 C. and skimming the resulting aluminum-copper compound from the surface of the bath. In this way the copper content may be reduced to 0.1% or less.
Aluminum-silicon coatings prepared in accordance with the invention are useful as corrosion-resistant coatings and also as brazes or solders for the application of aluminum protec-tive sheathing to the base metal.
In the application of aluminum-silicon coatings either for use as such or for use as brazing alloys, it is sometimes desirable to thoroughly degas the alloy bath by passing a mixture of 20% chlorine and 80% nitrogen, by volume, through the bath for a few minutes and then to modify it by the addi-tion of a small proportion, for example about 0.1%, of metallic sodium. Unless these expedients are employed the aluminum-silicon coating may exhibit gas pockets and a coarse grain which reduce its effectiveness both as a protective coating and as a bonding medium. The coarse grain becomes more pronounced as the cooling period increases. The aluminum-silicon bath should be renewed often enough to prevent an excessive tin content.
Aluminum-silicon coatings of the type described have been employed for the protection of cylindrical uranium rods from about 1 inch to about 1xc2xdinch in diameter. For the complete protection of such rods, the ends may be capped by aluminum caps or ferrules which may be compressed over the ends of the rods by suitable dies. Preferably the ends of the rod are machined down the thickness of the cap so that with the cap in place a smooth exterior surface is provided. Circumferential ridges over which the ferrules can be compressed may be left at or near the ends to assist in holding the cap in place. The most advantageous moment for the application of the ferrules is after the certrifuging of the rod to remove excess tin. Before applying the ferrules, it is advisable to dip the ends of the rod into molten aluminum-silicon in order to secure the maxium degree of adherence. By immediately applying the caps or ferrules to the hot coated ends and then dipping the assembly in molten aluminum-silicon alloy, a firmly adherent bond between the rod and the ferrules is obtained. If desired, the ferrules may be designed to provide an insulating gas space at the ends of the rod and a spacer of steel, beryllium, or some other metal may be inserted to reinforce the cap. It is sometimes convenient to employ a very thick metal cap so as to improve the conductivity at the end of the rod.
Aluminum-silicon coatings may be built in thickness by placing the capped and coated rod on smooth steel or asbestos-cement rollers and pouring molten alloy into the trough formed between the rod and one of the rollers. Pure aluminum also may be applied over the aluminum-silicon in this matter.
The bronze, tin, aluminum-silicon sequence of coatings is especially advantageous for the bonding of aluminum sheaths or cans to uranium rods. The procedure can be carried out in the same manner as described previously up to the point of rolling the coating on rollers.
One satisfactory canning method involves placing an aluminum can in a steel supporting shell in a canning die mantained at a constant and uniform temperature between about 590xc2x0 and 625xc2x0 C., pouring a small quantity of molten aluminum-silicon alloy into the can and plunging the uranium rod, immediately upon withdrawal from the aluminum-silicon bath, into the can so as to force the molten aluminum-silicon in the can out through the space between the rod and walls of the can. The quantity of aluminum-silicon in the can should be adjusted appropriately for the diameter and length of the rod canned and the clearance between rod and can. For rods about 1.4 inch in diameter and about 8 inches in length with about 15 mils total clearance between rod and can, about 60 to 100 grams have been found to be very suitable. Most of the aluminum-silicon applied by the hot-dip is washed from the rod by the molten metal in the can. Hence by em-ploying an aluminum-silicon alloy of high purity in the can, a low tin content may be secured even though the alloy applied by hot dipping was relatively impure in this respect. It is not necessary to provide ferrules on the rods to protect the ends when they are to be protected by an aluminum can. It is usually preferred to employ an aluminum can having a thick bottom to facilitate radial heat flow at the end (during ulti-mate use of the product), and to insert a thick aluminum disc in the top of the can to serve a similar purpose and also form a closure for the can. This disc should be preheated in an inert medium to a temperature in the neighborhood of that employed for the canning operation. The excess aluminum-silicon in the can serves as a brazing alloy to braze the cap into the can. Preferably the canned rod is promptly quenched. Quenching may be effected either before or after removing the canned rod from the steel supporting shell. Prompt quench-ing limits the dissolving action of the aluminum-silicon alloy on the bronze coating and on the uranium, and in this way assists in preventing formation of the brittle compound layer previously described. It also insures a fine-grained alloy.
The canning process described permits the complete assembly of a uranium rod in a can of 20 to 35 mil wall thickness in a total elapsed time of as little as 45 seconds. Operating in this manner the reaction of the base metal with the aluminum-silicon alloy is controlled to limit the thickness of the aluminum-uranium-silicon compound layer to less than 0.3 mils and this layer, to the extend it is formed, is free from detrimental cracks of fissures.